Light Emitting Diodes
A light emitting diode (LED) is a diode that converts electrical energy directly into light. When forward biased, electrons and holes recombine at the PN junction and release energy as photons. The color of the light depends on the semiconductor materials used to make the junction. LEDs are everywhere in ham radio equipment: power indicators, S-meter bar graphs, panel backlights, SWR bargraph displays, and status lights on every modern transceiver and accessory.
- How an LED produces light
- Forward voltage and current
- The current-limiting resistor
- LED colors and their forward voltages
- LEDs in ham radio
How an LED Produces Light
An LED is a PN junction diode made from semiconductor compounds such as gallium arsenide (GaAs), gallium phosphide (GaP), or indium gallium nitride (InGaN). When forward biased and current flows, electrons from the N side and holes from the P side recombine at the junction. In these special semiconductor materials, recombination releases energy in the form of photons — light — rather than heat as in silicon. The energy of the photon determines its color (wavelength), and that energy is set by the bandgap of the semiconductor compound.
Unlike incandescent bulbs, LEDs do not have a filament and do not produce significant heat. They convert electrical energy to light very efficiently and last for tens of thousands of hours. This is why LEDs have completely replaced indicator bulbs in modern equipment.
Forward Voltage and Current
Like all diodes, an LED has a forward voltage drop (Vf) — the voltage that appears across it when conducting. Unlike silicon rectifier diodes (0.6–0.7 V), LEDs have higher forward voltages that vary by color. The more energetic the photon (shorter wavelength = bluer light), the higher the forward voltage required.
| Color | Typical Vf | Semiconductor |
|---|---|---|
| Infrared | 1.2 – 1.5 V | GaAs |
| Red | 1.8 – 2.2 V | AlGaAs, GaAsP |
| Amber / Orange | 2.0 – 2.2 V | GaAsP |
| Yellow | 2.1 – 2.4 V | GaAsP, InGaAlP |
| Green | 2.0 – 3.5 V | GaP, InGaN |
| Blue | 2.5 – 3.7 V | InGaN |
| White | 3.0 – 3.5 V | InGaN + phosphor |
Most standard LEDs have a rated forward current (If) of around 10–20 mA. Running them at higher current produces more light but shortens their life and generates heat. Running them at lower current (1–5 mA) often produces sufficient brightness for an indicator and significantly extends their life.
The Current-Limiting Resistor
An LED must never be connected directly across a voltage source — it has very low internal resistance and would immediately draw destructive current. You must always connect a series resistor to limit the current to the LED's rated value.
The series resistor drops the voltage difference between the supply and the LED's forward voltage, setting the current. R = (Vsupply − Vf) / If.
View LargerThe formula comes directly from Ohm's law. The resistor must drop the remaining voltage (supply minus LED forward voltage), and the current through the resistor is the same as the current through the LED (series circuit):
R = (Vsupply − Vf) / If
- Vsupply = supply voltage
- Vf = LED forward voltage (from datasheet or table above)
- If = desired LED current in amps (e.g. 0.010 for 10 mA)
LED Current Limiting Resistor Calculator
Calculate the series resistor needed to run an LED safely. Enter supply voltage, LED forward voltage, and desired current.
⚖ Experiment: Build and Verify an LED Circuit
Build the classic LED indicator circuit, verify the current-limiting resistor calculation, and observe what happens without it.
- 9 V battery
- Red LED (Vf ≈ 2.0 V)
- 470 Ω resistor (calculated for about 15 mA at 9 V)
- Multimeter
- Breadboard and wires
- Calculate the expected current: R = (9 V − 2.0 V) / 0.015 A = 467 Ω. Use the nearest standard value: 470 Ω.
- Build the circuit on a breadboard: 9 V battery positive → 470 Ω resistor → LED anode → LED cathode → battery negative.
- Measure the voltage across the LED with the multimeter. It should read approximately 2.0 V.
- Measure the voltage across the resistor. Calculate the current: I = VR / 470. It should be close to 15 mA.
- Now carefully try a 10 kΩ resistor instead and note the change in brightness. The LED will be much dimmer (about 0.7 mA) but still works — showing that LEDs can be used at well below their rated current as low-brightness indicators.
With 470 Ω: LED glows brightly, voltage across LED approximately 2.0 V, current approximately 15 mA. With 10 kΩ: LED barely visible but still lit, confirming LEDs can operate over a wide current range. The voltage across the LED changes very little (it is diode-like, not ohmic), while the resistor voltage changes a lot — confirming that the resistor is doing the current-setting work.
LEDs in Ham Radio
LEDs appear throughout ham radio equipment and accessories:
- Power indicators: The on/off power LED on every transceiver, power supply, and accessory.
- S-meter and bar graphs: Rows of LEDs driven by a voltage comparator chain display received signal strength or power output in a quick-to-read bargraph format.
- SWR indicators: Forward and reflected power bargraphs show antenna match quality.
- PTT and transmit indicators: Red LED indicators warn that the transmitter is active.
- Backlit panels: White or blue LEDs illuminate front panel displays, scales, and knobs in modern equipment.
- Optical isolation: Infrared LEDs inside optocouplers transfer signals between circuits without any electrical connection — useful for audio isolation and keying circuit interfaces.
Frequently Asked Questions
Can I connect an LED directly to a battery without a resistor?
No. An LED has very low internal resistance when forward biased — without a series resistor, the current is limited only by the battery's internal resistance and the wiring resistance. This is almost certainly far more current than the LED is rated for. The LED will likely burn out almost instantly, sometimes with a visible flash and pop. Always use a current-limiting series resistor.
Why do LEDs of different colors have different forward voltages?
The color of light is determined by the energy of each photon. Blue photons have higher energy than red photons. The energy released when an electron-hole pair recombines equals the bandgap of the semiconductor material. Higher-energy photons (blue, white) require a larger bandgap, which means more voltage must be applied to forward bias the junction. Red LEDs have the lowest forward voltage; blue and white LEDs have the highest.
What happens if I put two LEDs in series on the same resistor?
In series, the forward voltages add. Two red LEDs in series drop about 4.0 V total (2 × 2.0 V). Recalculate the resistor using the combined Vf: R = (Vsupply − Vf1 − Vf2) / If. Both LEDs pass the same current and glow at the same brightness. This is a common way to run multiple LEDs from a single resistor when the supply voltage is high enough.
Test Your Knowledge
Answer the questions below to check your understanding. Every answer can be found in the lesson above.